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ArticleTitle Towards a better understanding of the Oulmes hydrogeological system (Mid-Atlas, Morocco) Article Sub-Title
Article CopyRight Springer-Verlag
(This will be the copyright line in the final PDF) Journal Name Environmental Earth Sciences
Corresponding Author Family Name Wildemeersch
Particle
Given Name Samuel
Suffix
Division Hydrogeology and Environmental Geology, GEO3, ArGEnCo Organization University of Liege
Address Building B52/3 Sart-Tilman, Liege, 4000, Belgium
Email swildemeersch@ulg.ac.be
Author Family Name Orban
Particle
Given Name Philippe
Suffix
Division Hydrogeology and Environmental Geology, GEO3, ArGEnCo Organization University of Liege
Address Building B52/3 Sart-Tilman, Liege, 4000, Belgium
Division Aquapôle
Organization University of Liege
Address Liege, Belgium
Author Family Name Ruthy
Particle
Given Name Ingrid
Suffix
Division Hydrogeology and Environmental Geology, GEO3, ArGEnCo Organization University of Liege
Address Building B52/3 Sart-Tilman, Liege, 4000, Belgium Email
Author Family Name Grière
Particle
Given Name Olivier
Suffix Division
Organization G2H Conseils
Address 29 Rue Blanche Hottinguer, Guermantes, 77600, France Email
Given Name Philippe
Suffix Division Organization
Address 1C Avenue du Léman, Thonon-Les-Bains, 74200, France Email
Author Family Name El Youbi
Particle
Given Name Abdelkhalek
Suffix Division
Organization Parc Industriel de Bouskoura, Eaux Minérales d’Oulmès
Address Casablanca, Morocco
Author Family Name Dassargues
Particle
Given Name Alain
Suffix
Division Hydrogeology and Environmental Geology, GEO3, ArGEnCo Organization University of Liege
Address Building B52/3 Sart-Tilman, Liege, 4000, Belgium Email
Schedule
Received 13 December 2008
Revised
Accepted 25 September 2009
Abstract Located in the Mid-Atlas (Morocco), the Oulmes plateau is famous for its mineral water springs “Sidi Ali” and “Lalla Haya” commercialised by the company “Les Eaux minérales d’Oulmès S.A”. Additionally, groundwater of the Oulmes plateau is intensively exploited for irrigation. The objective of this study, essentially performed from data collected during isotopic (summer 2004) and piezometric and hydrogeochemical field campaigns (spring 2007), is to improve the understanding of the Oulmes hydrogeological system. Analyses and interpretation of these data lead to the statement that this system is constituted by a main deep aquifer of large extension and by minor aquifers in a perched position. However, these aquifers interact enough to be in total equilibrium during the cold and wet period. As highlighted by isotopes, the origin of groundwater is mainly infiltration water except a small part of old groundwater with dissolved gas rising up from the granite through the schists.
Keywords (separated by '-') Hydrogeological characterisation - Hydrogeochemistry - Isotopes - Morocco Footnote Information
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O R I G I N A L A R T I C L E 1
2
Towards a better understanding of the Oulmes hydrogeological
3
system (Mid-Atlas, Morocco)
4 Samuel Wildemeersch•Philippe Orban•
5 Ingrid Ruthy•Olivier Grie`re•Philippe Olive•
6 Abdelkhalek El Youbi•Alain Dassargues
7 Received: 13 December 2008 / Accepted: 25 September 2009
8 ÓSpringer-Verlag 2009
9 Abstract Located in the Mid-Atlas (Morocco), the Oul-10 mes plateau is famous for its mineral water springs ‘‘Sidi 11 Ali’’ and ‘‘Lalla Haya’’ commercialised by the company 12 ‘‘Les Eaux mine´rales d’Oulme`s S.A’’. Additionally, 13 groundwater of the Oulmes plateau is intensively exploited 14 for irrigation. The objective of this study, essentially per-15 formed from data collected during isotopic (summer 2004) 16 and piezometric and hydrogeochemical field campaigns 17 (spring 2007), is to improve the understanding of the 18 Oulmes hydrogeological system. Analyses and interpreta-19 tion of these data lead to the statement that this system is 20 constituted by a main deep aquifer of large extension and 21 by minor aquifers in a perched position. However, these 22 aquifers interact enough to be in total equilibrium during 23 the cold and wet period. As highlighted by isotopes, the 24 origin of groundwater is mainly infiltration water except a 25 small part of old groundwater with dissolved gas rising up 26 from the granite through the schists.
27 Keywords Hydrogeological characterisation
28 Hydrogeochemistry Isotopes Morocco
29 Introduction
30 All over the world, water resources are under serious
31 pressure and their efficient management and protection
32 constitute a key issue for the future. This is especially the
33 case in country with semi-arid zones where such resources
34 are strongly exploited for irrigation. The first step in order
35 to manage and protect groundwater resources consists in
36 understanding precisely the local hydrogeological system.
37 It can be particularly uneasy to reach this objective in
38 fractured hard rock geological formations.
39 The Oulmes plateau, located in the Mid-Atlas
(Mor-40 occo), with its famous ‘‘Sidi Ali’’ and ‘‘Lalla Haya’’
min-41 eral water springs, is a particularly challenging case study.
42 The main objective of this work is to improve the
under-43 standing of the hydrodynamic and hydrogeochemical
pro-44 cesses occurring in a very complex geological and
45 hydrogeological system located in a contrasting climate
46 zone with increasing man-induced pressures. These
pres-47 sures, including groundwater extraction for
commerciali-48 sation and irrigation as well as use of fertilizers for
49 farming, are suspected of modifying the hydrogeological
50 system in terms of quantity and quality, which justifies the
51 necessity of such a scientific study.
52 Previous private and unpublished hydrogeological
53 studies, including local piezometric surveys and
hydrog-54 eochemical studies, lead to a conceptual and general
55 statement that the groundwater pumped in wells owned by
56 the company comes from a mixing of shallow groundwater
57 (with the resulting possible contamination), intermediate
58 groundwater fairly mineralised and deep mineralised
A1 S. Wildemeersch (&) P. Orban I. Ruthy A. Dassargues
A2 Hydrogeology and Environmental Geology, GEO3, ArGEnCo,
A3 University of Liege, Building B52/3 Sart-Tilman,
A4 4000 Liege, Belgium
A5 e-mail: swildemeersch@ulg.ac.be
A6 O. Grie`re
A7 G2H Conseils, 29 Rue Blanche Hottinguer,
A8 77600 Guermantes, France
A9 P. Olive
A10 1C Avenue du Le´man, 74200 Thonon-Les-Bains, France
A11 A. El Youbi
A12 Parc Industriel de Bouskoura, Eaux Mine´rales d’Oulme`s,
A13 Casablanca, Morocco
A14 P. Orban
A15 Aquapoˆle, University of Liege, Liege, Belgium
Environ Earth Sci
DOI 10.1007/s12665-009-0312-1
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59 groundwater with dissolved gas in variable proportions 60 linked to the granitic intrusion. However, these previous 61 studies lacked data and could not be considered as 62 exhaustive enough to produce results with a high degree of 63 certainty. The present study, focused mainly on the ‘‘Sidi 64 Ali’’ spring, is based on a larger set of data collected during 65 field campaigns carried out during summer 2004 and spring 66 2007. Among others, these campaigns produced measured 67 groundwater levels and groundwater samples from many 68 springs and wells. Temperature and precipitations data 69 ranging from October 1985 and September 2006 were also 70 collected in the ‘‘Baˆa’’ farm located at immediate prox-71 imity of the ‘‘Sidi Ali’’ spring. Thanks to this large set of 72 data, a comprehensive hydrogeological study including 73 hydrodynamic, hydrogeochemical, and isotopic parts was 74 undertaken providing the results described here below. 75 Study zone
76 The Oulmes plateau (Mid-Atlas, Morocco), located 77 150 km southeast of Rabat, is entirely included in the
78 Moroccan central plateau (Termier 1936; Beaudet 1969;
79 Michard1976). The plateau is limited by the Afcal wadi to
80 the north, the Boulahmayal wadi to the south and to the
81 west, and a main road in the east (Agard et al. 1950)
82 (Fig.1). The altitudes of the Oulmes plateau range from
83 450 m (in the valley of the Boulahmayal wadi) to 1,250 m
84 (near the city of Oulmes) while the most frequent altitude is
85 1,080 m (Dadi 1998).
86 The contrasting climate is characterised by a warm and
87 dry season from May to September, and a cold and wet
88 season from October to April. The mean annual
tempera-89 ture and precipitations, calculated for the 1985–2006
per-90 iod, are respectively 16.9°C and 635 mm. About 90% of
91 precipitations are recorded during the cold and wet period.
92 Except the Afcal and the Boulahmayal wadis, all others
93 wadis are not perennial and they flow only during the wet
94 season.
95 Water resources of the Oulmes plateau are
inten-96 sively exploited. Mineral waters from the ‘‘Sidi Ali’’
97 and ‘‘Lalla Haya’’ springs are both commercialised by
98 the company ‘‘Les Eaux Mine´rales d’Oulme`s S.A’’,
99 respectively, under the names of ‘‘Sidi Ali’’ and
Fig. 1 Location of the Oulmes plateau and the study zone
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100 ‘‘Oulme`s’’ but groundwater is also exploited for inten-101 sive irrigation to cultivate various fruits species in 102 modern orchards producing early fruits for Europe. 103 Additionally, private and public wells of the ‘‘Office 104 National de l’Eau Potable’’ (ONEP) located all over the 105 zone but mainly near the town of Oulmes produce 106 water supply for local inhabitants.
107 Geological and hydrogeological settings 108 Geological setting
109 Geologically, the Oulmes plateau is located at the northeast 110 of the Khouribga-Oulmes anticlinorium and it includes two 111 main types of geological formations: a central plutonitic
112 granite formation (Fig.2, A) surrounded by schists
for-113 mations (Fig. 2, B). Eastwards from these schists
forma-114 tions, quartzitic formations (Fig. 2, C) are outcropping
115 together with shales with limestone and sandstone nodules
116 (Fig.2, D) and limestones (Fig.2, E).
117 The Oulmes granite outcrops on about 30 km2with an
118 elliptic form oriented NNE–SSW with a large axis of
119 8.5 km and a small axis of 4.25 km (Diot et al.1987). This
120 plutonitic intrusion occurred 298 My ago (Gmira 1996)
121 along a subvertical sinistral shear zone oriented N10E and
122 generated during the Hercynian tectonic phase (Diot et al.
123
1987; Gmira1996). This phase, characterised by a regional 124 compression oriented NW–SE (Lagarde 1985; Diot et al.
125
1987), folded and faulted the zone which had previously 126 been folded by the Ante-Visean tectonic phase (Lagarde
127
1985; Tahiri1991).
Fig. 2 Geological schema and cross-section of the Oulmes plateau showing mainly the Oulmes granite and the Palaeozoic formations [modified
from (Baudin et al.2001)]
Environ Earth Sci
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128 The Palaeozoic schists formations, predominantly Ordo-129 vician, are the results of the metamorphism induced in the 130 formations surrounding the granitic intrusion. This metamor-131 phism, locally hydrothermal, increases as the rocks are close to 132 the granitic intrusion (Michard1976; Boutaleb1988; Gmira 133 1996). The most metamorphised zones are the andalusite zone 134 and the biotite zone. Beyond these zones, less metamorphised 135 zones are found which are the green mica zone and the chlorite 136 zone (Boutaleb1988; Baudin et al.2001).
137 Eastwards, the quartzitic formations form a main topo-138 graphical ridge of the plateau. Towards the east and beyond 139 these erosion resistant formations, Silurian and Devonian 140 shales with limestone and sandstone nodules are found. In 141 the extreme northeast of the zone, Visean limestones are 142 observed in unconformity with the former formations 143 because of the Ante-Visean tectonic phase. Consequently, 144 the Tournaisian formations are missing on the Oulmes 145 plateau. Volcanic and superficial formations such as the 146 pinkish granitic gravely sands arena, resulting from the 147 granite alteration and lying on a wide outcropping surface 148 with a mean thickness of 0.6 m (Agard et al.1950), are 149 also present in the zone.
150 Hydrogeological setting
151 The main hydrogeological units of the Oulmes plateau can
152 be considered as: (1) the superficial formations,
charac-153 terised by a granular porosity, and (2) the fissured granitic
154 and schists formations, essentially characterised by a
155 fracture porosity.
156 The superficial formations, resulting from the granite,
157 schists and quartzites weathering constitute very thin shallow
158 aquifers. The granitic and schists formations are considered as
159 aquifers only when a significant hydraulic conductivity is
160 induced by interconnected fissure networks. The main
mea-161 sured fracture directions of the case study zone are orientated
162 N10–N30 and N120–N140 and they are mostly subvertical
163 (Dadi1998). Pumping tests performed in the case study zone
164 provide an average hydraulic conductivity of 10-5m/s.
165 The ‘‘Sidi Ali’’ spring is currently exploited and bottled
166 by the company ‘‘Les Eaux Mine´rales d’Oulme`s S.A’’
167 thanks to wells drilled in metamorphised schists very close
168 to the Oulmes granite (andalusite zone). Consequently, the
169 original ‘‘Sidi Ali’’ spring does not flow anymore, proving
170 that pumpings have already modified the original
Fig. 3 Piezometric map of the study zone in March 2007 (isoline interval = 10 m)
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171 hydrogeological system. The company owns also produc-172 tion wells on the ‘‘Sidi Brahim’’ site located to the east 173 from the ‘‘Sidi Ali’’ site (Fig.1).
174 Materials and methods
175 Three kinds of data were collected for this study: isotopic 176 data, hydrogeochemical data and piezometric data. 177 Groundwater samples used for isotopic analyses were 178 collected during a field campaign carried out in summer 179 2004. These isotopic samples were taken from 15 wells and 180 springs chosen at different altitudes and located over the 181 whole Mid-Atlas. The isotopic samples, representative of 182 small watersheds and representative of precipitations, were 183 analysed following standard procedures in Hydroisotop 184 GmbH laboratory in Schweitenkirchen (Germany). 185 Water levels measurements and sampling of ground-186 water for geochemical analyses were performed during a 187 field campaign in spring 2007. These 110 geochemical 188 samples were taken manually from springs, shallow and 189 deep wells without any pumping (except for three wells of 190 the company). The piezometric head were measured with a 191 piezometric probe and the hydrogeochemical analyses of 192 major ions were performed using standard techniques in 193 the laboratory of the University of Liege. The acceptable 194 error on the ion balance of each sample is taken at 5%.
195 Results
196 Piezometric map
197 The piezometric map of the study zone (Fig.3) was plotted
198 including both static heads measured in private wells and
199 dynamic heads measured in the three production wells of
200 the company producing at this period (March 2007).
201 The piezometric map indicates that the topography
202 strongly influences the piezometric heads which are
203 underlined by steeper hydraulic gradients near crests.
204 Groundwater, as surface waters, is drained by two main
205 wadis which are the Afcal northward and the Boulahmayal
206 southward and westward. Consequently, groundwater flow
207 occurs to the north and to the southwest to join,
respec-208 tively, the Afcal and the Boulahmayal wadis. Where
209 groundwater is not directly drained by these wadis, it is
210 drained by their main tributaries. Groundwater flow occurs
211 also from east to west along the principal ridge of the
212 Oulmes plateau which constitutes both a hydrological and a
213 hydrogeological limit. Influence of the pumping is clearly
214 visible through the cones of depression induced in Sidi Ali
215 and Sidi Brahim sites. Furthermore, the Sidi Brahim
216 pumping influences the hydrogeological system of the
217 ‘‘Sidi Ali’’ spring by capturing a part of the groundwater
218 that would flow naturally towards the ‘‘Sidi Ali’’ spring.
219 The ‘‘Sidi Ali’’ groundwater capture zone as delineated
220 from this piezometric map covers 10.94 km2.
221 Annual groundwater budget
222 An annual groundwater budget is established for the ‘‘Sidi
223 Ali’’ spring capture zone in order to estimate the infiltration
Table 1 Mean annual actual evapotranspiration (AEvT) and mean
annual available water (Wa) calculated using the Thornthwaite
method for three different values of maximum soil moisture storage
capacity (STOmax)
STOmax(mm) AEvT (mm) Wa(mm)
100 366 269
175 429 205
250 457 158
Fig. 4 Observed flow in a wadi during a heavy precipitations event
Table 2 Required infiltration surface (Srequired) calculated for the
three values of maximum soil moisture storage capacity (STOmax) and
the three distributions of the available water (Wa) into runoff (R) and
infiltration (I) R(mm) I(mm) Srequired(km2) STOmax=100 mm ? Wa=269 mm R=2/3 of Wa 179 90 2.98 R=3/4 of Wa 202 67 4.00 R=7/8 of Wa 235 34 7.88 STOmax=175 mm ? Wa=205 mm R=2/3 of Wa 137 68 3.94 R=3/4 of Wa 154 51 5.25 R=7/8 of Wa 179 26 10.31 STOmax=250 mm ? Wa=158 mm R=2/3 of Wa 105 53 5.06 R=3/4 of Wa 119 39 6.87 R=7/8 of Wa 138 20 13.40
Environ Earth Sci
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Table 3 Temperature ( T in °C), electrical conductivity (EC in l S/cm), pH and concentrations in main chemical species (in mg/l) of each groundwater sample collected during the field campaign Well ID Lithological unit T ( °C) EC (lS/cm) pH a Ca 2 ? (mg/l) Mg 2 ? (mg/l) Na ? (mg/l) K ? (mg/l) Fe (mg/l) Cl -(mg/l) SO 4 2 -(mg/l) NO 2 -(mg/l) NO 3 -(mg/l) CO 3 2 -(mg/l) HCO 3 -(mg/l) CO 2 (mg/l) PB1 3 19.2 386.00 8.28 27.09 10.48 35.73 2.32 0.49 22.86 65.18 B 0.200 3.01 1.04 98.95 0.40 PB2 3 18.0 334.00 8.40 24.25 10.07 30.78 1.35 1.31 13.89 51.98 B 0.200 B 0.300 1.51 108.95 0.30 PB3 3 17.7 319.30 7.91 27.77 9.32 22.20 2.47 2.10 12.48 33.07 B 0.200 15.22 0.51 113.52 1.00 PB5 3 17.9 374.00 8.36 26.29 9.28 37.58 1.64 1.59 21.56 58.56 B 0.200 B 0.300 1.38 109.25 0.30 PB6 3 17.5 443.00 8.11 32.03 11.49 42.30 1.58 0.26 21.95 91.83 B 0.200 1.46 0.73 103.28 0.60 PB8 3 18.1 376.00 7.99 24.47 10.48 33.53 2.52 17.30 19.05 89.43 B 0.200 1.51 0.37 68.71 0.50 PB11 3 16.8 444.00 8.26 46.08 15.68 22.59 1.96 1.38 15.74 78.32 B 0.200 0.92 1.45 144.49 0.60 PB12 3 15.4 316.00 8.20 18.98 9.93 30.26 2.33 1.83 14.60 52.78 B 0.200 B 0.300 0.82 93.34 0.40 PB17 3 13.9 219.90 8.07 16.83 5.80 18.43 2.62 16.40 7.40 22.71 B 0.200 B 0.300 0.59 90.17 0.50 PB20 3 15.9 173.80 7.66 10.05 4.42 18.05 0.86 1.44 12.29 18.48 B 0.200 8.08 0.11 44.87 0.70 PB24 3 17.6 403.00 8.15 39.34 12.90 25.04 1.35 0.38 13.07 88.62 B 0.200 B 0.300 0.82 105.53 0.50 PB25 3 17.1 417.50 8.32 43.98 11.51 25.48 1.85 0.58 12.21 74.87 B 0.200 0.72 1.57 135.71 0.50 PB26 3 16.6 371.00 7.99 30.46 10.92 25.37 1.62 1.31 11.23 106.00 B 0.200 1.49 0.30 55.42 0.40 PB27 3 17.3 300.20 8.33 26.02 11.99 18.91 1.48 1.27 11.17 18.92 B 0.200 0.86 1.68 142.78 0.50 PB28 3 16.6 354.00 8.16 37.01 11.24 21.01 2.09 2.08 9.35 23.96 B 0.200 1.17 1.39 173.88 0.80 SA 2 17.8 239.30 7.76 16.68 7.06 21.81 1.38 4.11 12.31 37.38 B 0.200 B 0.300 0.26 82.37 1.00 Pz5 2 20.7 230.10 7.63 12.15 6.88 21.91 3.57 5.75 12.58 35.42 B 0.200 B 0.300 0.16 69.14 1.10 S11 1 13.6 120.00 7.45 6.82 1.84 13.42 1.24 0.14 7.86 3.65 B 0.200 19.70 0.05 30.36 0.80 S13 2 20.3 139.00 7.45 4.43 2.17 19.47 1.42 0.07 10.20 3.70 B 0.200 21.47 0.05 32.80 0.80 F4 2 19.0 302.20 7.44 14.78 9.97 26.50 2.89 19.10 13.99 87.84 B 0.200 1.24 0.05 31.58 0.80 F6 2 17.7 350.00 6.80 21.21 11.75 24.56 3.10 0.63 13.72 128.09 B 0.200 2.91 B 0.050 B 0.050 0.00 F7 2 18.2 267.70 7.82 13.73 7.41 26.41 3.42 1.86 24.11 41.85 B 0.200 0.83 0.22 60.48 0.60 F8 3 18.9 338.00 7.87 20.08 12.19 28.76 2.62 5.53 13.43 82.87 B 0.200 B 0.300 0.31 74.93 0.70 F9 2 19.0 269.30 7.92 12.38 8.56 28.25 2.86 6.59 14.33 47.06 B 0.200 B 0.300 0.36 77.24 0.60 X2 3 15.8 166.50 7.87 10.90 5.39 16.26 0.93 0.23 7.64 19.10 B 0.200 3.43 0.26 62.86 0.60 X4 1 12.7 127.00 7.33 7.30 1.71 13.52 1.33 14.50 8.68 4.16 B 0.200 21.76 0.04 30.42 1.00 X7 1 17.3 613.00 7.65 36.65 11.73 66.18 5.82 0.15 71.92 44.57 B 0.200 108.88 0.13 54.57 0.90 X10 3 14.8 174.30 6.38 11.64 6.85 7.55 1.12 0.18 10.43 18.60 B 0.200 48.68 B 0.050 B 0.050 0.00 X13 1 18.6 157.10 7.83 15.19 1.24 13.69 2.00 0.20 6.99 3.04 B 0.200 21.69 0.20 54.44 0.60 X3 1 14.0 165.50 7.61 9.12 2.38 19.02 1.05 0.26 13.39 2.88 B 0.200 25.79 0.09 37.62 0.60 X5 1 13.2 133.70 7.53 7.14 1.80 16.40 0.99 1.07 7.93 2.71 B 0.200 22.05 0.07 37.62 0.80 X9 3 13.5 187.10 4.50 5.67 8.02 10.51 2.65 0.77 15.16 51.07 B 0.200 1.83 B 0.050 B 0.050 0.00 X11 3 17.4 165.30 7.39 8.70 4.88 15.16 0.76 0.94 12.12 20.90 B 0.200 16.11 0.04 30.36 0.90 X12 3 18.2 402.00 7.76 47.59 11.32 22.15 4.63 1.45 11.66 24.90 B 0.200 5.51 0.64 199.85 2.40 X14 1 16.5 158.30 7.35 10.65 2.81 17.43 1.44 0.40 6.81 3.20 B 0.200 19.24 0.07 59.56 1.90Author Proof
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Table 3 continued Well ID Lithological unit T ( °C) EC (lS/cm) pH a Ca 2 ? (mg/l) Mg 2 ? (mg/l) Na ? (mg/l) K ? (mg/l) Fe (mg/l) Cl -(mg/l) SO 4 2 -(mg/l) NO 2 -(mg/l) NO 3 -(mg/l) CO 3 2 -(mg/l) HCO 3 -(mg/l) CO 2 (mg/l) X15 1 16.2 144.60 7.36 16.82 1.35 9.93 2.36 0.42 6.43 4.89 B 0.200 3.40 0.09 68.10 2.10 X16 2 15.4 129.40 7.01 10.19 2.07 10.74 1.74 0.18 7.08 2.26 B 0.200 27.94 0.02 30.42 2.10 X17 2 18.5 399.00 7.88 39.42 9.26 30.72 6.16 0.22 27.42 8.64 B 0.200 12.31 0.72 171.62 1.60 X19 2 18.2 471.00 8.36 47.55 9.85 26.14 23.09 0.15 24.64 36.37 B 0.200 2.75 2.43 192.47 0.60 X21 1 15.5 268.10 7.51 20.34 5.03 20.33 3.76 0.10 18.17 11.16 B 0.200 66.85 0.06 31.58 0.70 X22 2 18.5 215.10 8.02 17.73 6.06 17.99 1.44 1.10 9.68 14.58 B 0.200 B 0.300 0.56 97.55 0.70 X23 3 17.7 227.60 8.06 30.95 5.15 6.59 1.04 2.41 7.49 30.09 B 0.200 B 0.300 0.54 85.41 0.50 X25 3 – 117.50 5.95 5.66 5.16 7.20 0.53 0.33 11.85 22.28 B 0.200 9.44 B 0.050 B 0.050 0.00 X26 4 – 168.80 7.44 11.81 7.35 6.62 1.71 0.25 13.28 1.85 B 0.200 36.38 0.05 30.36 0.80 X27 5 – 432.80 8.22 73.54 9.58 5.99 B 0.050 0.41 7.09 50.30 B 0.200 4.18 1.82 198.63 0.80 SB1 3 18.5 226.50 7.81 20.29 6.18 17.57 1.10 1.44 9.46 25.31 B 0.200 B 0.300 0.33 91.94 1.00 SB3 3 19.8 248.90 7.76 26.49 5.66 17.13 B 0.050 4.61 9.19 21.86 B 0.200 B 0.300 0.35 110.23 1.30 SB4 3 18.8 208.20 7.45 14.00 5.07 17.83 4.32 0.74 10.33 26.49 B 0.200 1.87 0.11 71.70 1.80 P1 2 17.5 148.90 6.95 6.62 2.37 18.31 1.42 0.61 8.19 2.91 B 0.200 29.54 0.02 36.52 2.90 P35-S3 1 12.8 256.20 7.58 23.73 4.97 19.61 1.57 0.05 25.57 12.99 2.02 19.04 0.14 64.32 1.20 P10 2 18.0 161.30 7.42 12.76 2.79 13.57 2.40 0.04 7.69 18.13 0.47 14.47 0.06 42.55 1.10 P11 2 15.9 148.70 7.54 18.69 2.01 7.07 2.29 0.03 5.68 1.85 B 0.200 24.63 0.10 53.41 1.10 P12 2 18.9 210.50 6.68 9.51 6.13 17.64 2.42 0.48 10.23 64.13 B 0.200 6.86 B 0.050 B 0.050 0.00 P13 2 16.5 213.80 7.50 10.76 5.74 20.45 4.55 0.46 15.17 29.02 B 0.200 10.62 0.09 51.03 1.10 P14 2 19.1 152.20 7.55 10.83 7.26 6.52 3.88 3.10 8.21 2.33 B 0.200 10.53 0.13 67.98 1.30 PB15 3 17.3 401.00 8.00 48.04 10.81 19.94 1.59 21.20 11.79 57.36 B 0.200 3.07 0.88 159.12 1.10 P17 2 17.3 238.30 7.57 15.66 7.81 19.77 0.96 0.24 19.18 15.11 B 0.200 13.38 0.16 80.11 1.50 P18 3 17.1 181.20 7.61 12.67 5.56 15.73 1.44 3.58 9.58 14.35 B 0.200 B 0.300 0.18 78.89 1.40 P19-S5 3 13.8 228.50 7.49 17.24 7.73 14.33 1.70 0.74 12.36 35.81 B 0.200 11.73 0.09 54.69 1.20 P20 2 18.0 202.90 7.72 15.85 5.96 18.26 1.33 1.30 10.32 14.45 B 0.200 B 0.300 0.27 93.28 1.20 P26b 4 16.1 250.90 7.95 38.83 6.68 6.80 0.99 1.76 8.98 2.02 B 0.200 3.86 0.69 139.98 1.10 P28 1 20.9 1173.00 7.56 88.13 21.24 103.23 4.86 0.17 156.19 33.40 B 0.200 279.39 0.10 49.75 1.00 P30 1 17.7 862.00 8.01 64.90 17.01 94.04 5.74 0.14 86.72 135.69 B 0.200 38.90 0.86 151.81 1.00 P31 1 18.8 1684.50 8.40 81.92 23.51 288.74 16.90 0.11 238.89 62.72 B 0.200 60.88 6.95 501.51 1.40 P32 1 17.7 773.30 8.21 80.84 6.57 75.94 10.28 0.21 57.49 78.07 B 0.200 61.45 1.99 222.65 1.00 P34 1 15.0 540.00 7.81 29.09 7.48 69.72 3.38 0.22 45.29 85.46 B 0.200 55.82 0.22 62.92 0.70 P36 1 15.3 227.30 7.29 15.09 5.24 17.10 3.86 0.31 24.90 6.12 1.16 80.90 B 0.050 B 0.050 0.00 P37 2 18.3 140.20 7.41 13.72 1.75 10.16 2.03 0.04 6.33 2.16 B 0.200 26.04 0.06 41.34 1.10 P40 3 16.5 244.50 8.00 27.50 5.40 16.13 1.17 0.94 8.00 19.63 B 0.200 2.94 0.62 112.06 0.80 P43 1 18.2 116.90 7.26 7.33 1.38 10.44 3.53 B 0.005 10.50 1.76 B 0.200 41.65 B 0.050 B 0.050 0.00 P45 3 19.0 188.90 7.63 22.45 5.49 5.67 2.37 8.41 8.92 18.08 B 0.200 6.36 0.16 66.70 1.10Environ Earth Sci
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Table 3 continued Well ID Lithological unit T ( °C) EC (lS/cm) pH a Ca 2 ? (mg/l) Mg 2 ? (mg/l) Na ? (mg/l) K ? (mg/l) Fe (mg/l) Cl -(mg/l) SO 4 2 -(mg/l) NO 2 -(mg/l) NO 3 -(mg/l) CO 3 2 -(mg/l) HCO 3 -(mg/l) CO 2 (mg/l) P46 3 16.6 149.30 7.71 13.89 3.71 9.69 3.74 0.03 6.89 14.00 B 0.200 1.64 0.17 60.60 0.80 P47 3 15.0 125.60 5.84 6.29 5.02 6.89 1.06 0.53 9.82 20.50 B 0.200 20.12 B 0.050 B 0.050 0.00 P48 3 16.6 309.90 3.41 16.32 10.81 11.14 4.20 1.39 8.42 108.85 B 0.200 B 0.300 B 0.050 B 0.050 0.00 P49 3 16.2 146.20 5.00 9.10 3.69 9.17 2.00 0.79 11.75 37.08 B 0.200 6.21 B 0.050 B 0.050 0.00 X43 3 16.3 146.90 7.24 9.59 5.37 B 0.020 2.59 2.59 7.94 25.86 B 0.200 11.91 B 0.050 B 0.050 0.00 P64 1 16.3 230.90 10.00 28.86 2.22 12.83 3.00 0.85 9.45 9.83 B 0.200 45.88 14.90 27.01 0.00 P69 5 – 147.20 7.49 15.34 4.33 7.60 0.77 4.89 5.24 22.50 B 0.200 B 0.300 0.09 51.03 1.20 P74 4 – 150.60 7.66 13.12 6.78 7.05 0.85 1.83 5.55 22.32 B 0.200 B 0.300 0.14 53.35 0.80 P79 2 16.5 459.00 7.94 56.00 7.50 18.37 11.56 3.60 14.68 51.10 B 0.200 74.51 0.46 95.35 0.80 P100 3 18.3 259.50 8.12 22.55 9.80 19.75 1.32 7.79 10.29 18.44 B 0.200 0.79 0.94 128.52 0.70 P101 4 16.0 230.70 7.87 21.67 9.26 11.19 1.00 0.17 4.94 47.11 B 0.200 1.76 0.30 72.49 0.70 P105 4 16.4 151.80 7.67 14.96 4.80 8.72 0.77 1.65 6.41 20.25 B 0.200 6.14 0.13 50.91 0.80 P106 4 16.5 143.00 7.53 11.14 4.95 8.90 0.97 B 0.005 7.04 4.38 B 0.200 28.69 0.07 38.84 0.80 P107 5 – 390.00 8.12 55.44 13.18 7.64 2.81 3.39 10.30 30.81 B 0.200 6.46 1.37 188.57 1.00 P108 5 – 226.80 7.92 36.92 3.47 3.68 1.06 5.10 6.99 16.62 B 0.200 28.01 0.36 78.46 0.70 PB21 3 16.2 463.00 8.13 48.69 15.20 27.59 1.22 2.86 14.99 102.10 B 0.200 1.48 1.02 136.87 0.70 X29 3 – 249.20 8.09 25.28 5.80 19.75 1.90 9.68 10.39 20.02 B 0.200 B 0.300 0.82 120.23 0.70 X30 3 – 210.50 8.03 18.59 4.26 16.35 3.39 0.07 10.24 21.67 B 0.200 5.47 0.48 80.66 0.50 X31 3 – 303.80 7.75 29.02 12.76 8.06 5.99 9.21 6.90 74.42 B 0.200 5.62 0.24 77.49 1.00 X33 2 – 249.70 7.65 11.51 8.46 25.43 2.47 4.02 23.26 18.03 B 0.200 B 0.300 0.23 92.18 1.40 X34 2 – 371.00 8.06 36.26 7.62 29.30 4.41 0.26 20.73 25.16 B 0.200 7.22 1.02 161.26 1.00 X35 2 – 295.90 7.85 12.90 10.28 29.72 2.25 1.68 36.12 16.14 B 0.200 2.76 0.37 94.31 0.90 X36 2 – 267.20 7.76 11.23 9.07 28.86 1.34 0.23 30.38 11.51 B 0.200 11.38 0.27 85.96 1.00 X37 2 – 290.70 7.81 12.02 9.30 32.45 1.83 0.17 32.23 13.05 B 0.200 16.38 0.33 93.16 1.00 X38 2 – 389.50 7.65 17.22 13.70 37.17 3.05 12.30 63.43 21.56 B 0.200 B 0.300 0.24 95.84 1.50 X39 2 – 378.00 7.90 14.62 12.47 39.39 2.58 1.86 61.09 22.24 B 0.200 B 0.300 0.39 88.16 0.80 X40 2 – 369.00 7.65 15.94 13.19 34.40 2.41 11.70 59.56 21.96 B 0.200 B 0.300 0.22 88.52 1.40 X41 2 – 306.80 8.01 31.71 6.29 22.79 2.71 0.50 19.22 28.72 B 0.200 3.85 0.68 120.47 0.80 X42 2 – 355.00 7.69 33.37 13.56 10.33 5.23 0.11 31.29 62.89 B 0.200 12.18 0.18 66.70 1.00 P51 – 283.40 8.09 27.84 10.07 17.24 0.98 1.49 10.96 14.68 B 0.200 0.80 1.00 146.68 0.80 P114 2 – 182.80 7.63 7.35 5.20 19.41 0.81 B 0.005 9.16 26.96 B 0.200 20.56 0.09 37.62 0.60 SX1 2 – 180.90 7.70 8.45 7.01 14.58 1.04 0.07 15.33 12.37 B 0.200 25.25 0.11 39.99 0.60 SB2 3 – 249.00 8.00 25.73 6.44 16.36 0.75 12.90 8.33 26.97 B 0.200 B 0.300 0.61 110.90 0.80 a pH measured in laboratoryAuthor Proof
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224 surface required to meet the total volume of outflows. 225 Afterwards, the risk of overdraft of groundwater resources 226 was evaluated comparing this surface with the ‘‘Sidi Ali’’ 227 spring capture zone surface.
228 Precipitations and evapotranspiration
229 The mean annual rainfall calculated from daily precipi-230 tations measured between October 1985 and September 231 2006 is about 635 mm. On the basis of a temperature 232 dataset available for the same period, the potential 233 evapotranspiration is estimated using the simple Thorn-234 thwaite formula which only requires mean monthly tem-235 perature, latitude and month (Thorntwaite and Mather 236 1955; Thorntwaite and Mather 1957). According to this 237 formula which is reasonably accurate in determining 238 mean annual values of potential evapotranspiration (Fetter 239 2001), the mean annual potential evapotranspiration is 240 calculated about 879 mm.
241 The annual actual evapotranspiration (AEvT) is then 242 calculated using the Thornthwaite method providing also 243 annual available water (Wa) which is the sum of runoff (R)
244 and infiltration (I). The main conceptual assumption of this 245 method is that infiltration can only occur after subtracting
246 the needed water for evapotranspiration from a soil water
247 storage capacity limited to a given maximum value
248 (STOmax). Thus, an empirical evaluation of this STOmaxis
249 required. This maximum soil moisture storage capacity
250 depends on many factors among which climate, vegetation
251 and soil types. The values proposed in the literature
252 (Re´me´nie´ras 1986; de Marsily 1994) vary from 10 to
253 20 mm for a 30-cm thick sandy soil, about 100 mm for a
254 30-cm thick silty to clayey soil and up to 300 mm for a thin
255 soil in arid zones. As the geology and the geomorphology
256 of the zone are complex, a wide variety of soil types are
257 found around the ‘‘Sidi Ali’’ spring. They are mainly
258 composed of sand on the weathered granite while they are
259 mainly composed of silt and clay on the weathered schists.
260 Additionally, the soil thickness in the study zone can vary
261 from 0 to 80 cm and locally up to 120 cm. Applying the
262 Thornthwaite method, a sensitivity analysis is performed
263 using several values for the maximum soil moisture storage
264 capacity: 100, 175 and 250 mm. It confirms that the
265 groundwater recharge occurs during the cold and wet
266 period ranging from October to April. As the soil moisture
267 storage does not reach its maximum during the driest years,
268 it consequently appears that no groundwater recharge can
269 be calculated during these years. The mean annual actual
270 evapotranspiration and the mean annual available water
271 calculated for the three values of the maximum soil
272 moisture storage capacity are given in Table1.
273 Infiltration
274 Considering the lack of stream hydrograph data, runoff (R)
275 and infiltration (I) are assessed from the calculated
avail-276 able water (Wa). The hilly topography, the significant
277 runoff and the observed quick response of the wadis to
278 heavy precipitations (Fig.4) lead to consider the runoff as
279 systematically larger than the infiltration. Three ratios were
280 chosen: R = 2/3 of Wa, R = 3/4 of Waand R = 7/8 of Wa.
281 Outflows
282 There is no groundwater pumped by the ONEP in the
283 capture zone of the ‘‘Sidi Ali’’ spring and wells.
Ground-284 water pumped by the inhabitants of the Oulmes plateau can
285 be considered as negligible in comparison with volumes
286 pumped by the company for bottled mineral water and the
287 Baˆa fruit farm for irrigation. The annual volume pumped
288 for both uses (Vout), estimated from a dataset of 10 years, is
289 about 268,000 m3 among which about one-third for
irri-290 gation. Any return flow, i.e. infiltration induced by the
291 irrigation is not considered since drip irrigation is used to
292 avoid losses.
Table 4 Evolution of the groundwater depth-averaged temperature depending on the total well depth
Total well depth (depthtot) Mean depth-averaged
temperature (°C)
SD (°C)
DepthtotB10 m (17 wells) 15.4 1.9
10 m \ depthtotB30 m (36 wells) 17.2 1.4
Depthtot[30 m (27 wells) 17.8 1.6
Fig. 5 Groundwater depth-averaged electrical conductivity distribution Environ Earth Sci
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293 Evaluation of possible overdraft
294 The term overdraft is defined as pumping in excess of the 295 average annual recharge (Fetter 2001) which means that 296 the sustainable extraction rate, groundwater withdrawal 297 that satisfies water supply requirements without deleteri-298 ous hydrogeological or environmental impacts (Loa´iciga 299 2008), is exceeded. According to this definition, the risk 300 of overdraft of the capture zone of the ‘‘Sidi Ali’’ spring 301 and production wells is very simply estimated by com-302 paring the annual infiltration with the annual pumped 303 volumes:
Srequired¼Vout I 305
305 where, Srequired is the required infiltration surface to meet
306 the total volume of outflows. This equation does not con-307 tain any baseflow term because there is almost no baseflow 308 in the case study zone (non-perennial wadis). The mean 309 annual runoffs, infiltrations and corresponding required 310 infiltration surfaces are given in Table2 for the different 311 considered scenarios.
312 The required infiltration surfaces obtained (Table2)
313 compared to the capture zone of the ‘‘Sidi Ali’’ spring and
314 wells evaluated from the piezometric map (10.94 km2)
315 (Fig.3) show that a risk of overdraft is expected in case of
316 successive dry years when infiltration is poor or even
317 nonexistent. According to the known volumes pumped
318 currently, the critical infiltration under which it can be
319 considered that groundwater pumpings are creating an
320 overdraft is 24.45 mm/year.
321 Hydrogeochemistry
322 Temperature (T in °C), electrical conductivity at 20°C (EC
323 in lS/cm) and concentrations in major ions (mg/l) of each
324 groundwater sample with an acceptable error on the ion
325 balance (maximum 5%) are given in Table3. As almost all
326 the samples were collected without any pumping, values
327 are considered as representative of depth-averaged
328 conditions.
329 The mean depth-averaged groundwater temperature in
330 the study zone is 17.0°C. The corresponding SD is 1.8°C.
331 A clear and logical dependence of the depth-averaged
Fig. 6 Division of the study zone into five lithological units
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332 temperature on the total well depth is observed. As shown 333 in Table4, the highest depth-averaged temperatures cor-334 respond to the deepest wells.
335 The mean depth-averaged groundwater electrical con-336 ductivity at 20°C in the study zone is 316 lS/cm. The 337 corresponding SD is 225 lS/cm. This high value of SD is 338 predominantly caused by two huge electrical conductivity 339 values measured in two granitic arena wells and equal to 340 1,183 and 1,570 lS/cm. High concentrations in Na? 341 (respectively, 103.2 and 288.7 mg/l ), Cl- (respectively, 342 156.2 and 238.9 mg/l) and NO3-(respectively, 279.4 and
343 60.9 mg/l) can explain these anomalous values. The 344 analysis of the depth-averaged groundwater electrical 345 conductivity distribution (Fig.5) indicates that this 346 parameter was lower than 200 lS/cm for about 30% of 347 the samples and lower than 400 lS/cm for more than 80% 348 of the samples. Consequently, groundwater from the 349 capture zone of the ‘‘Sidi Ali’’ spring and wells is overall 350 poorly mineralised.
351 Chemical species
352 The groundwater samples are classified into five classes 353 according to the lithological units from where they were
354 sampled in the study zone (Fig.6): unit 1—granite and
355 granitic arena (A in Fig.2); unit 2—highly metamorphised
356 schists (andalusite and biotite zones) (part of B in Fig. 2);
357 unit 3—poorly metamorphised schists (green mica and
358 chlorite zones) (part of B in Fig.2); unit 4—quartzites (C
359 in Fig.2); unit 5—shales with limestone and sandstone
360 nodules (D in Fig.2). Results in terms of major ions are
361 shown in a Piper (Piper 1944) diagram (Fig.7) for
362 underlining possible correlation between the lithology and
363 the observed hydrogeochemical facies.
364 In the cation triangle of the Piper diagram, it can be
365 observed that 18% of the samples are of Ca-type, 70%
366 show no dominant cation type and 12% are Na ? K-type.
367 The distribution of the samples into the different cation
368 types depending on the lithological units is given in
369 Table5.
370 Although statistics made on units 4 and 5 could hardly
371 be considered as completely representative (not enough
372 samples), a correlation between cation type and lithological
373 unit is indicated in this distribution (Table5). Unit 1
374 samples show mainly waters in Na ? K-type which can be
375 related to the dissolution of feldspars such as albite and
376 orthose contained in the granite but also waters in the
Ca-377 type and in the no dominant cation type. The transition to
378 units 2, 3 and 4 samples is characterised by a gradual
Fig. 7 Piper diagram of groundwater samples classified by lithological units and showing hydrogeochemical facies
Environ Earth Sci
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379 enrichment in Ca2?and Mg2?against Na?
and K?
leading 380 in majority to the no dominant cation type. Unit 5 samples 381 are entirely included in the Ca-type waters which can be 382 related to the dissolution of limestone nodules contained in 383 the shales.
384 The anion triangle indicates that 63% are of the samples 385 HCO3?CO3-type, 13% are of the SO4-type, 4% are
Cl-386 type and 20% show no dominant anion type. The distri-387 bution of the samples into anion types depending on the 388 lithological units is given in Table6.
389 As for the cations and despite of few exceptions in units 390 4 and 5, a correlation between the dominant anion type and 391 the lithological unit is suggested in this distribution 392 (Table6). Unit 1 samples show waters of the 393 HCO3?CO3-type, Cl-type and no dominant anion type.
394 The transition to units 2 and 3 samples is characterised by a 395 gradual enrichment of groundwater in SO42- against Cl
-396 and, in a minor way, against HCO3
-and CO32-, which can
397 be related to the dissolution of pyrite favouring the SO4
-398 type. Samples of units 4 and 5 are entirely belonging to the
399 HCO3?CO3-type of water related to the dissolution of
400 the limestone nodules contained in the shales.
401 Finally, the groundwater hydrogeochemical facies
402 shown in the Piper diagram are Ca–Mg–HCO3, Ca–Mg–
403 SO4–Cl, Na–K–Cl and Na–K–HCO3for, respectively, 57,
404 27, 7 and 9% of the samples. The two main facies are thus
405 Ca–Mg–HCO3 and Ca–Mg–SO4–Cl since they include
406 84% of the samples. The distribution of the groundwater
407 samples into the different hydrogeochemical facies
408 depending on the lithological unit is given in Table7.
409 The main facies is Ca–Mg–HCO3 (Table7) but a
410 change in groundwater geochemical composition
depend-411 ing on the lithological units is clearly visible as the facies
412 gradually evolves from Ca–Mg–HCO3to Ca–Mg–SO4–Cl
413 and then to Na–K–HCO3or Cl.
414 Although only few samples were available for units 4
415 and 5, mean concentrations in main chemical species
416 depending on the lithological units were calculated. They
417 are given in Table8. A Stiff (Stiff1951) plot showing the
418 mean groundwater chemical composition in each
litho-419 logical unit is also given (Fig.8).
420 The high mean concentrations in Na?, Cl-and NO3
-in 421 unit 1 samples underline anthropogenic effects. A severe
422 contamination in nitrates is indeed observed in most of unit
423 1 samples with concentrations generally larger than 50 mg/
424 l and locally exceeding 275 mg/l. The use of fertilizers can
425 explain this contamination especially in the granitic arena
426 where the sampled wells are most often shallow and
427 located in a very permeable coarse sand grain-size
forma-428 tion. The concentrations in nitrates in units 2 and 3 waters
429 are probably lower because the sampled wells in the schists
430 are deeper and located in a lower permeable formation
431 where reducing redox conditions induced nitrates
reduc-432 tion. This is confirmed by the very low concentrations
433 (most often \1 mg/l) in nitrates measured in samples
col-434 lected in the irrigation wells of the Baˆa farm and in the
435 production wells of the company ‘‘Les Eaux Mine´rales
436 d’Oulme`s S.A’’.
Table 5 Distribution of the groundwater samples into cation types depending on the lithological units
Unit Number
of samples
Samples in cation type (%)
Ca Na ? K No cation dominant 1 18 22 45 33 2 33 9 12 79 3 43 16 0 84 4 6 17 0 83 5 4 100 0 0
Table 6 Distribution of the groundwater samples in anion types depending on the lithological units
Unit Number of samples
Samples in anion type (%)
HCO3?CO3 SO4 Cl No anion dominant 1 18 61 0 22 17 2 33 64 9 0 27 3 43 53 26 0 21 4 6 100 0 0 0 5 4 100 0 0 0
Table 7 Distribution of the groundwater samples in hydrogeochemical facies depending on the lithological units
Unit Number of
samples
Samples in hydrogeochemical facies (%)
Ca–Mg–HCO3 Ca–Mg–SO4–Cl Na–K–HCO3 Na–K–Cl
1 18 28 28 33 11 2 33 55 18 12 15 3 43 61 40 0 0 4 6 100 0 0 0 5 4 100 0 0 0
Author Proof
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437 Isotopes
438 The content in oxygen-18 (d18O in %) and hydrogen-2 (dD 439 in %) of samples collected over the whole Mid-Atlas and 440 in a zone surrounding the ‘‘Sidi Ali’’ spring are given in 441 Table9.
442 d18O
443 The oxygen-18 isotope allows evaluating the recharge
444 altitude from the regional altitude isotopic gradient (Olive
445
1996). The regional altitude isotopic gradient of Oulmes is 446 estimated to -0.22 % per 100 m (Fig. 9) from samples
447 collected in 15 springs and wells located over the whole
448 Mid-Atlas region at different altitudes ranging from 300 to
449 2,100 m.
450 According to d18O concentrations, the main recharge
451 altitude of groundwater surrounding the ‘‘Sidi Ali’’ spring
452 would be about 1,200 m. Accordingly, the main recharge
453 would occur around the principal fractured quartzitic ridge
454 of the study zone located in the east and including several
455 summits above 1,200 m.
456 d18O and dD
457 A comparison between the oxygen-18 isotope and the
458 hydrogen-2 isotope allows to highlight isotopic exchanges
459 modifying the composition of sampled groundwater in
Fig. 8 Stiff plot of the mean groundwater chemical composition in each lithological unit Table 8 Mean concentrations in major ions depending on the
litho-logical units
Unit 1 Unit 2 Unit 3 Unit 4 Unit 5
Number of samples 18 33 43 6 4 Ca2?(mg/l) 30.6 18.0 23.6 18.6 45.3 Mg2?(mg/l) 6.6 7.5 8.4 6.6 7.6 Na? (mg/l) 49.0 22.1 18.8 8.2 6.2 K? (mg/l) 4.1 3.5 2.0 1.1 1.2 Fe total (mg/l) 1.1 2.5 3.6 0.9 3.5 HCO3 -(mg/l) 82.5 74.9 84.9 64.3 129.2 CO32-(mg/l) 1.4 0.3 0.6 0.2 0.9 SO42-(mg/l) 28.1 28.0 43.0 16.3 30.1 Cl -(mg/l) 44.6 20.5 11.8 7.7 7.4 NO3-(mg/l) 55.2 11.5 4.6 12.8 9.7
Environ Earth Sci
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460 oxygen-18 isotope (Olive1996; Akouvi et al. 2008). The 461 local meteoric water line (LMWL) (Fig.10) of the region 462 is estimated to be: dD = 8 d18O ? 9 %. Very similar to 463 the World Meteoric Water Line (WMWL) defined by Craig 464 (1957), this linear relation with a gradient of eight indicates 465 that the chemical composition of sampled groundwater was 466 not significantly modified by evaporation.
467 Graphically, the position of the samples surrounding the
468 ‘‘Sidi Ali’’ on the left of the LMWL suggests an isotopic
469 exchange which modifies their isotopic composition. This
470 one can only occur between CO2 and H2O via the
equi-471 librium C16O2?H218O$ C16O18O ? H216O. The
corre-472 sponding equilibrium fractioning factor at 100°C is:
e18OCO 2H2O¼ 18O=16O CO2 18O/16O ½ H2O ¼ 1; 030 474 474 This fractioning indicates CO2 enriching in 18O against
475 H2O. This enrichment, estimated to 1% for the Oulmes
476 plateau groundwater, requires high concentrations in CO2
477 suggesting a magmatic origin. The possibility of CO2rising
478 up from the granite through the fissured schists was
con-479 firmed by a borehole drilled in the highly metamorphised
480 schists (andalusite zone) where CO2gas was measured at a
481 concentration of 2 g/l. 482 13 C and14C 483 The carbon-13 and carbon-14 isotopes allow separating
484 total dissolved carbon (Ctot) into carbon from biological
485 origin (Cbio) and carbon from mineral (magmatic) origin
486 (Cmin) (Olive 1996). Standard values of Cbio and Cmin in
487 terms of 13C and 14C concentrations are given in
488 Table10.
489 The 13C content measured in the carbonates of highly
490 metamorphised schists is -9.6 %. The 13C and 14C
con-491 centrations in samples collected in wells of the company
492 ‘‘Les Eaux Mine´rales d’Oulme`s S.A’’ surrounding the
493 ‘‘Sidi Ali’’ spring are given in Table11.
494 Considering these values and the corresponding graph
495 (Fig.11), Cmin would constitute about 30–50% of Ctot in
Table 9 Oxygen-18 (d18O in %) and hydrogen-2 (dD in %) content
in samples from the whole Mid-Atlas (A–Y) and from a zone sur-rounding the ‘‘Sidi Ali’’ spring (F5 to Tam Ahamat)
ID Altitude (m) d18O (%) dD (%) A 897 -6.24 -38.9 AA 2,140 -9.02 -62.0 B 698 -5.75 -35.9 E 284 -4.99 -30.7 G 795 -5.80 -36.6 H 1,166 -6.61 -42.7 I 1,000 -6.21 -39.3 J 1,485 -8.13 -54.1 L 1,711 -8.15 -55.9 O 1,668 -8.03 -56.7 Q 1,619 -6.90 -45.5 U 439 -4.71 -26.8 V 352 -4.72 -29.6 W 1,281 -6.86 -42.5 X 1,451 -7.08 -45.9 Y 1,205 -8.45 -55.8 F5 1,030 -7.60 -44.0 Pz5 1,030 -7.70 -42.0 SA 1,062 -6.58 -41.5 Ain Karrouba 1,050 -6.96 -41.4 Tam Ahamat 850 -6.76 -43.4
Fig. 9 Altitude isotopic gradient of the Oulmes plateau
Fig. 10 Local Meteoric Water Line (LMWL)
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496 groundwater surrounding the ‘‘Sidi Ali’’ spring which 497 confirms once more that gas is rising up from the granite 498 through the schists.
499 Another process likely to produce CO2is decarbonata-500 tion at high temperature. This possibility is negligible in 501 this case study since there is almost no carbonate in the 502 highly metamorphised schists (Table10). Additionally, if a 503 decarbonatation process producing a CO2 enriched of 1– 504 2 % in13C, was active, the13C concentration measured in 505 Lalla Haya should be about -8 % instead of the measured 506 -3.5 % (Table 10). Consequently, such a process can be 507 considered as inactive in the study zone.
508 14C and3H
509 Carbon-14 isotope and hydrogen-3 isotope allow estimat-510 ing the mean age of groundwater (Olive 1996; 2000). 511 Although there is magmatic CO2 gas rising up, ground-512 water from the study zone should be relatively young since 513 14C concentrations measured in samples collected in wells 514 of the company ‘‘Les Eaux Mine´rales d’Oulme`s S.A’’ 515 (Table11) indicate that groundwater surrounding the ‘‘Sidi 516 Ali’’ spring is younger than 200 years. On the other hand, 517 the 3H concentrations measured in the same wells 518 (Table12) suggest that groundwater surrounding the ‘‘Sidi 519 Ali’’ spring is older than 50 years since only younger 520 groundwater (or mixing of old and young groundwater) 521 would contain more than 2 TU (Fig.12). According to14C 522 and 3H concentrations, groundwater from the case study 523 zone should be aged from about 50 to 200 years.
524 Discussion
525 About piezometry (Fig.3), this study confirms the general
526 trends observed on the partial piezometric maps obtained in
527 the less extensive previous studies: the piezometric
528 depression in the centre of the study zone and the
529 groundwater flow direction westwards along the eastern
530 slope of the principal ridge. According to this piezometric
531 map, water recharge can mainly occur in the upward zone
532 at an altitude of the plateau between about 1,175 and
533 1,250 m. The saturated zone is reached at an altitude
534 between approximately 1,150 and 1,200 m. A confirmation
Fig. 11 13C and 14C concentrations in C
bio, Cmin and wells
surrounding the ‘‘Sidi Ali’’ spring
Table 12 3H concentration in
wells surrounding the ‘‘Sidi Ali’’ spring
Well ID T(TU)
F5 B1.9
Pz5 B2.4
Fig. 12 3H concentration measured in precipitations in
Thonon-les-Bains since 1950
Table 11 Measured13C and14C concentrations in wells surrounding
the ‘‘Sidi Ali’’ spring
Well ID 13C (%) 14C (pCm)
F5 -11.7 48.9 ± 0.8
F6 -16.5 77.6 ± 1
Pz5 -11.9 45.1 ± 0.6
Table 10 13C and14C contents (standard values) in biological carbon
(Cbio) and mineral carbon (Cmin)
13
C (%) 14C
(pCm)
Cbio -22 100
Cmin -3.5a 0
Carbonates of the highly metamorphised schists -9.6 –
a 13
C concentration measured on gas in Lalla Haya by Winckel (2002)
Environ Earth Sci
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PROOF
535 is provided by the isotope analyses indicating that a mean 536 recharge altitude of about 1,200 m (Fig.8) corresponding 537 to the upward part of the catchment and to the main ridge 538 of the Oulmes plateau constituted by fractured quartzites. 539 A specific discussion point must be addressed about the 540 possible partitioning of the considered aquifer in low pie-541 zometric conditions. Static piezometric heads measured in 542 some wells close to each others are quite different during 543 the irrigation period (a difference of 3 m at a distance of 544 30 m) while they are nearly similar (i.e. in equilibrium) in 545 winter. This observation confirms that in dry and irrigation 546 conditions, the hydrogeological system can be considered 547 as functioning with two separated aquifers: a deep and 548 large extension aquifer and shallow and small capacity 549 perched aquifer compartments. Conversely, after winter 550 recharge, respective piezometric heads in the regional deep 551 aquifer and in the shallow compartments cannot be 552 distinguished.
553 Clear correlations between hydrogeochemical facies and 554 lithology are demonstrated despite a high heterogeneity in 555 groundwater chemical composition in each lithological 556 unit. Although the Ca–Mg–HCO3 groundwater facies 557 remains predominant, it progressively evolves along the 558 main flow path of the study zone since the Ca–Mg–HCO3,
559 Ca–Mg–SO4–Cl and Na–K–HCO3 or Cl facies appear,
560 respectively, in the fractured quartzites, the poorly meta-561 morphised schists and the highly metamorphised schists 562 and the granite and granitic arena (Table7).
563 As interpreted from the isotope analyses, groundwater 564 from the capture zone of the ‘‘Sidi Ali’’ spring and wells 565 results from a mixing of ‘‘young’’ water coming from the 566 recharge with ‘‘old’’ water with dissolved gas rising up 567 from the granite through the schists. A schema summaris-568 ing the hydrogeological functioning of the ‘‘Sidi Ali’’ 569 spring according to the results obtained in this study is 570 proposed in Fig.13.
571 Conclusion
572 A new hydrogeological study, performed from a large set
573 of recent data mainly collected in a zone surrounding the
574 ‘‘Sidi Ali’’ spring, allows to improve the conceptual
575 understanding of the complex Oulmes fractured
hydro-576 geological system. This system, located in schists
meta-577 morphised by a surrounding granite, appeared to be
578 constituted by a main deep aquifer of large extension and
579 by minor aquifers with a small capacity in a perched
580 position with regards to the main deep aquifer. Clearly
581 separated during the dry and irrigation period, the deep and
582 the shallow aquifers interact enough to be in total
equi-583 librium after the winter recharge. The origin of
ground-584 water is mainly infiltration water except, as highlighted by
585 isotopes, a small part of old groundwater with dissolved
586 gas rising up from the granite through the schists.
587 Piezometric and isotopic data show that the recharge
588 occurs mainly at an altitude of about 1,200 m
corre-589 sponding to the principal ridge of the study zone made of
590 fractured quartzites. Furthermore, a main flow path starts
591 from this ridge to go westwards to the centre of the plateau.
592 Although the main hydrogeochemical facies observed in
593 the Piper diagram is Ca–Mg–HCO3, an evolution of the
594 groundwater facies is visible along this main flow path
595 since Ca–Mg–SO4–Cl and Na–K–HCO3 or Cl facies
596 appear when the schists and the granite and granitic arena
597 are reached.
598 An intense exploitation of groundwater resources,
599 especially for mineral water bottling and irrigation, could
600 lead to overdraft only in case of successive dry years when
601 the winter recharge is poor or nonexistent.
602 Concerning the quality of groundwater, a severe
con-603 tamination in nitrates is observed in wells drilled in the
604 granite and the granitic arena. This contamination is not
605 observed in the schists, probably because nitrates are
Fig. 13 Schema of the hydrogeological functioning of the ‘‘Sidi Ali’’ spring
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606 reduced in this anoxic media. However, this last statement 607 can be biased by the fact that most of the wells drilled in 608 the schists are deeper and more protected than those drilled 609 in the granite and the granitic arena.
610 Acknowledgments Authors are grateful to the company ‘‘Les Eaux
611 Mine´rales d’Oulme`s S.A’’ for supporting this study and the needed
612 field campaigns.
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